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Selected metallic platings and insulating coatings on steel fasteners were evaluated for ability to reduce galvanic corrosion of die cast magnesium in a modified salt spray test. Proprietary electroplate systems based on zinc, zinc-nickel, zinc-cobalt and tin-zinc were tested, along with a commercial 50-50 tin lead alloy electroplate without supplementary coating. A proprietary liquid-applied zinc-rich inorganic coating successfully used on automotive fasteners was also tested for compatibility with magnesium. Encapsulation of bolt heads with plastic insulating coatings or special molded caps was evaluated. Interruption of the continuous salt spray by rinse and bake cycles was investigated as a likely exposure condition for magnesium assemblies in powertrain or underhood applications. Several of the protection schemes were found to effectively eliminate galvanic corrosion of the magnesium.

The compatibility of magnesium die casting alloys with commercial automatic transmission fluids was studied in laboratory tests. The effects of high temperature, presence of water, and galvanic coupling with steel were examined, using visual observation, weight change, Scanning Electron Microscope, and Scanning Auger Microprobe surface analysis. No significant corrosion of magnesium was detected under any of the test conditions.

Magnesium alloys are being used in a broad variety of structural applications in the automotive, aerospace, electronics, and consumer products industries. While the growth of these applications has been driven primarily by weight savings, other advantages of magnesium alloys have played an important role. A major advantage is their unique suitability to the die casting process, allowing high rate production of near-net shape parts. Along with superior die life, this yields production cost savings that can more than offset the raw material cost advantage of competitive die cast aluminum alloys. Excellent machinability and superior damping properties are other important properties of these alloys. An outstanding feature of the new generation of magnesium alloys is their superior resistance to salt water corrosion.

High purity die cast magnesium clutch housings and 4WD transfer cases have demonstrated their ability to function in corrosive road salt environments without the use of protective coatings. The weight reduction opportunities offered by magnesium also extend to highly visible components such as wheels, retractable headlight covers, and exterior mirror housings, where a decorative and durable finish is required. This paper examines the effects of chemical surface preparation on the performance of a typical automotive paint system on die cast AZ91D, in salt and fresh water laboratory exposures. The effects of prior mechanical damage to the coating by Gravelometer testing and other means are examined. Mild etching chrome pickles provided superior paint preparation for nominally clean as-cast surfaces. Commercial iron phosphates were effective when applied over previously pickled surfaces.